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Moon and Venus

September 8, 2013

The crescent Moon and the planet Venus stage a spectacular encounter in the fading twilight this evening. Venus — the brilliant “evening star” — is just a couple of degrees to the right of the Moon. They set by a couple of hours after sunset.

The Moon appears as a crescent because it’s lining up roughly between Earth and the Sun. Most of the lunar farside — the side that we never see from here on Earth — is basking in sunlight. But it’s nighttime on most of the lunar hemisphere that faces Earth — only a tiny sliver is seeing the Sun.

For Venus, though, it’s just the opposite. Venus passed “behind” the Sun as seen from Earth back in late March. Right now, the planet is aligned in such a way that it’s daylight for more than two-thirds of the hemisphere that faces our way, and nighttime for the rest of it. So if you look at Venus through a telescope, it looks like a gibbous Moon.

Over the next few months, though, Venus will loop between Earth and the Sun, so the illuminated fraction will grow smaller. It’ll go from a gibbous phase, to a “quarter” phase, and finally to a crescent — the same cycle as the Moon.

You might expect that to make Venus look fainter, but it doesn’t. That’s because Venus is also moving closer to Earth. The combination will actually make Venus look even brighter as we head through fall. It’ll reach its peak brightness for the entire year in early December.

We’ll talk about the Moon and another planet tomorrow.


Script by Damond Benningfield, Copyright 2013

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